1. Field of the Invention
The present invention relates to mechanisms for anchoring a well tool to a well casing and, more particularly, to such an anchoring mechanism that comprises an array of slips that are collectively set and which are individually engaged with the inside wall of the well casing.
2. Description of Related Art
It is well known that a packer creates, by its existence in a subterranean well, an annular volume between a well casing and a well tubing, and in some embodiments, is attached to the tubing as it is in inserted in the well. When the desired location in the well is reached during insertion, the packer is "set" by activating an anchoring mechanism commonly referred to as a "slip" (or in plurality "slips") to affix the packer to the well casing, and to compress a sealing member outwardly to seal against an inside diametrical wall of the well casing.
In some packers a hydraulically operated piston is integral to the anchoring mechanism, and utilizes hydraulic pressure applied to the tubing to move the slips into connective engagement with the well casing. Slips typically engage an interior surface of the well casing by a series of hardened teeth which lock the packer in position. Once the packer is set, the ability of the packer to resist movement and maintain a seal, despite the loads that may be imposed during normal operation of the well, is critical to successful operation of the packer and the safety of the well. Loads which are commonly incurred in a well may include tubing weight, wellbore pressure acting on the annular seal area, axial forces due to well pressure fluctuations and/or loads imposed by thermal expansion or contraction of the tubing. In deeper than average wells, the ability of the slips to resist movement is critically important. Some conventional packers employ a single concentric hydraulic piston acting in a single direction on a radial array of slips. The pressure used to set the packer acts on the area of the piston and is translated to an axial force, which in turn acts on an annular cone. The cone contacts a mating conical surface on the slips thereby causing the slips to move radially outward to engage the interior surface of the casing.
It is well known that additional pressure applied to set the packer causes a higher radial force at the slips, which results in a greater ability for the packer to resist the loads in the well. However, the amount of pressure that can be applied to set the packer is often limited by the pressure rating of the tubing. In other words, if a higher pressure is used to set the slips, the slips will deform the tubing. Further, additional axial force can be generated by increasing the piston area, but generally this cannot be done because the available annular area is constrained by the packer outside diameter and the tubing inside diameter.
When a single piston acts on a radial array of slips, lack of concentricity and misalignment can negatively effect packer performance. When one slip contacts the interior surface of the casing, the entire force of the hydraulic piston is transferred to that slip thereby limiting the effectiveness of the remaining slips in the array. This causes the packer to move when the loads are borne by the packer, which can cause the seal to be damaged or destroyed. This condition is only minimally improved by the use of a plurality of pistons since typically one piston acts in the upward direction on a single array of slips and one piston acts in the downward direction on a single array of slips.
There is a need for a device to intensify the setting pressure of the packer by bringing greater force to the slips without increasing setting pressure, and for each slip to be collectively set, but independently moved into connective engagement with the interior surface of the casing.